1. Field of the Invention
The present invention relates to a process for preparing succinic anhydride (SA) by catalytic hydrogenation of maleic anhydride (MA) in the liquid phase.
SA is a valuable synthetic building block for preparing succinic acid, diesters of succinic acid, succinimides, polyesters and pharmaceuticals.
2. Description of the Related Art
It is already known that SA can be obtained by dehydration of succinic acid or by catalytic hydrogenation of maleic anhydride (MA). The catalytic hydrogenation of MA is generally carried out in the liquid phase over a suspended catalyst.
Mild reaction conditions, the co-use of a diluent, the co-use of deactivators for the catalyst and the setting of low temperatures are provided here. These mild reaction conditions quite obviously serve to suppress undesired reactions such as the polymerization of the MA or overhydrogenation, for example to give .gamma.-butyrolactone. Thus, according to CS 218 083 (cited according to C. A. 103, 6210 k), the hydrogenation is carried out in ethyl acetate in the presence of Raney nickel which has previously been deactivated by acetic acid or by ethyl acetate.
For example, according to CS 195 860, a 1:1 mixture of MA in .gamma.-butyrolactone is reacted over catalysts from the group consisting of copper on kieselguhr, NiO on kieselguhr, Cu--Zn--Cr catalyst or Raney copper at from 100.degree. to 300.degree. C. to give a reaction product containing succinic acid, .gamma.-butyrolactone and propionic acid.
Here the C.sub.3 product propanol is formed from the C.sub.4 starting material MA, apparently by decarbonylation or decarboxylation.
According to JP 02/200 680 (cited according to C. A. 114, 42561 m), maleic acid or MA is reacted at 200.degree. C. in tetraethylene glycol dimethyl ether to give .gamma.-butyrolactone and SA; use is here made of a complicated catalyst system comprising ruthenium, organic phosphines, conjugated bases of acids having a pKa &lt;2 and chlorine-containing compounds from the group consisting of hydrochlorides or metal chlorides.
According to SU 1 541 210 (cited according to C. A. 113, 23679 u), the reaction is carried out in acetone as solvent at from 15.degree. to 50.degree. C. using a Pd/activated carbon catalyst which had been modified with triethylamine. Dioxane is proposed as solvent in SU 721 406 (cited according to C. A. 93, 71047 b), which solvent contains the MA as a from 30 to 5% strength by weight solution.
The process of JP 48/07609 (1973) is carried out in the gas phase with use of a further specific measure, namely a deficiency of hydrogen (molar ratio H.sub.2 : MA=1:3), with, according to Example 1 of this Japanese patent application, both Pd on Al.sub.2 O.sub.3 and a Ni--Re catalyst being used together as hydrogenation catalyst.
According to the process of CN 1 063 484, SA is prepared by melting MA at from 60.degree. to 80.degree. C. and carrying out the hydrogenation at a rising temperature of from 70.degree. to 126.degree. C. in the presence of a Raney nickel catalyst comprising four components (without more detailed description).
The reaction product is subsequently cooled and, after solidification, is broken up coarsely and packed. Although this process avoids the use of a solvent, it incorporates instead a complicated (not described in more detail) catalyst and the need for a complicated temperature profile which obviously follows the increasing formation of SA in the reaction mixture and, if it is not adhered to, results in SA crystallizing out and the stirrer being impeded.